Two forms of commercially useful high-vinyl polybutadiene include syndiotactic 1,2-polybutadiene and atactic 1,2-polybutadiene. Syndiotactic 1,2-polybutadiene is a high-vinyl polybutadiene that has a stereoregular structure in which the side-chain vinyl groups are located alternately on the opposite sides in relation to the polymeric main chain. Although syndiotactic 1,2-polybutadiene is a crystalline thermoplastic resin, which results from its stereoregular structure, it uniquely exhibits the properties of both plastics and rubber, and therefore it has many uses. For example, films, fibers, and various molded articles can be made from syndiotactic 1,2-polybutadiene. It can also be blended into and co-cured with natural or synthetic rubbers in order to improve the properties thereof. For example, syndiotactic 1,2-polybutadiene can be added to rubber stocks for the manufacture of tires.
Atactic 1,2-polybutadiene is an amorphous, rubbery elastomer that has a stereoirregular structure in which the side-chain vinyl groups are located randomly on the opposite sides in relation to the polymeric main chain. Atactic 1,2-polybutadiene is utilized in a variety of applications. For example, atactic 1,2-polybutadiene is useful in tire tread compositions because it provides a good balance of traction and rolling resistance.
Due to its high-vinyl structure, high-vinyl polybutadiene is prone to thermal crosslinking to form a gel, i.e., an insoluble and non-processable material. Unfortunately, after being synthesized, high-vinyl polybutadiene is often isolated and dried at high temperatures. Or, when used as an additive in the manufacture of tires, it is subjected to compounding temperatures that can reach 165–180° C. During the compounding, it is critical that high-vinyl polybutadiene is not gelled before it is mixed with other ingredients. Otherwise, good dispersion of the polymer in other ingredients cannot be achieved. Also, especially in the case of syndiotactic 1,2-polybutadiene, molding, such as injection or compression molding, typically takes place at temperatures higher than the melting point of the polymer, which can exceed temperatures of 100–200° C. Therefore, the stabilization of high-vinyl polybutadiene is very critical for its utilization.
Many antioxidants have been employed in the art to prevent problems such as thermal crosslinking. The selection of a useful antioxidant, however, is unpredictable, especially when the amount of antioxidant employed is a factor. For example, when high-vinyl polybutadiene is compounded with a rubber for the manufacture of tires, the type and amount of antioxidant employed can deleteriously impact vulcanization of the tire rubber. Further, antioxidants are costly and therefore it can be advantageous to employ a minimal amount of antioxidant.
Therefore, it would be advantageous to find effective stabilizers for preventing the thermal crosslinking of high-vinyl polybutadiene.